Computer assistance?

I need to basically "crash" my computer. My sisters downloaded a lot of things off of the web, like games and such and it really messed up my computer completely. I can't get into my Add or Remove Programs and I'm positive there are a lot of viruses and such swimming around on it... It's slowed my computer access majorly and I can't even get into some of my folders, so could someone give me some pointers?

this just happend to me
it really sucks what i did was signed up for McAfee and windows defender and i scaned my computer they found the virus which they then removed
and i sent it to McAfee so they can study it or whatever anyways i just signed up for their free trial program you dont have to give them a credit card number or anything so its really free ;)
but you might have to redownload somethings on your computer like windows etc it damaged alot of my files for internet explorer so i had to download new programs oh and if you have adobi reader and flas player you will have to redownload them as well hope this was helpful

AVG is a free virus program thing. Works really well. Also Adsense and Spy... detector will help too. I'd dload them and run them and see if it'll help a bit. If it's really bad you might have to reformat. Let's hope not.

This will clear EVERYTHING off your computer, including Windows (or whatever OS you use.) Make sure you have the installation disks for Windows, Microsoft Word, etc. and anything else you use.

Also be aware that if you have things on there you don't want to lose, like pictures and stuff, a good computer tech would probably be able to save those files and fix your computer without reformatting. It shouldn't cost too much, and that way you can be reimbursed if something goes wrong. If your computer is lost forever after you try and fix it yourself, there's not a lot anyone can do.

Be sure that after you reformat, you get a good spyware and virus program. I use Norton (although my subscription expired awhile ago) and SpyBot. Look at the settings and see if there's a way to have it notify you immediately when a virus is detected- I get a popup window.